Call callback without rank when fetching rank.

Changes the `rankFor` and `rankForIn` functions to call the callback even
for an error or if the member isn't found, similar to the behaviour for
`scoreFor`/`scoreForIn`.

Added separate unit test for ensuring the callback is triggered in the
case of an error for scoreFor* and rankFor*.
Always trigger callback during conditional ranking

If a member ranking is skipped do to the condition of ranking not being
met, the callback is still triggered, albeit with 0 return to indicate
no records were added, in the spirit of ZADD.
Always trigger callback for percentile*

Changes the percentile family of functions to always call the callback
even on error.
Always execute callback for rankedInList*
Always execute callback for aroundMe*
Merge pull request #2 from ludometrics/trigger-all-the-things

Trigger all the things